Grading process and apparatus



Feb. 14, 1956 J. J. KoBLlsKA 2,734,631

GRADING PROCESS AND APPARATUS Filed D66. 23, 1953 """ul I 2 l 3 M 4 if /2 mmm-Ilm!! t 'm i INVEN'vI'OR JOHN u. Magus/r4,

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ATTORN EY United jStates Patent GRADING PROCESS AND APPARATUS John I. Kobliska, Dnnellen, N. I., assignor to American Cyanamd Company, New York, N. Y., a corporation of Maine Application December 23, 1953, Serial No. 400,007

3 Claims. (Cl. 209-237) This invention relates to a wet process for grading particles and to an apparatus useful for carrying out the process. More particularly, the invention relates to a process and an apparatus therefore for testing samples of particulate material for the percentage of fines or coarse particles in the material by impinging a sheet of grading liquid on particles of the test material supported on a rotating sieve.

e The process of this invention is generally designated a wet grading method in the art. The word grading connotes the determination of the particle size distribution in a given sample of particulate material. As a rule, the value sought is the percent of particles in a-sample which are too large to pass through a given size of sieve opening. Manufacturers and users of pigments, for example, are often interested in the value mentioned because the color value or tinctorial power of a pigment depends to a large extent on the size of the pigment particles. Overa practical range of sizes, the ner the pigment particles, the higher the tinctorial power. There has been a need in the art for a process and apparatus for determining with reproducible results the percent of retention on a sieve of oversized pigment particles. A standard test recognized by both the pigment manufacturer and the user would eliminate much disagreement between the buyer and seller of the commodity when the analysts of the two parties nd different percent retentions for the same pigment sample by using the unreliable manual tests of the prior art.

Previously, particles have been graded by both dry and wet methods. rThe dry method commonly consists of placing a known quantity of the material to be tested on the uppermost and coarsest sieve of a nest of sieves and agitating the sieves either manually or mechanically. The material retained on each of the successively finer sieves is weighed and its percentage of the original sample calculated. Dry grading methods are generally slow, subject to error due to spillage and loss of material as flydust, and as a rule, give non-reproducible results because of the human errors introduced in the method. The human errors are the variation in the amount of agitation from one analyst to the next or even between diiferent experiments by the same analyst, and equally serious, the determination of the end point of the operation. It has been noted that analysts employed by pigment manufacturers tend to be more diligent in shaking and brushing the material on the sieves and are also apt to work for a longer period of time on a given sample. This, of course, is due to the analysts desire to nd as low a percent retention as possible for his employers product. The analyst employed by the pigment user or buyer has no particular desire to find a low percent retention and is, therefore, less diligent than the manufacturers analyst and is almost certain to get a different result. The dry method is also objectionable since the ily-dust settles on laboratory equipment creating a considerable nuisance in many cases.

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Wet methods of grading particles have been found to be somewhat more satisfactory than the dry methods described above. In the usual wet grading process, a weighed sample is brushed under a stream of water from a tap or other source through a nest of sieves as in the dry grading process. Thev mechanical losses due to spillage and loss of material through ily-dust are obviated. However, the human errors discussed above are also introduced here, so the wet grading method has not proven very satisfactory. Also, reproducible values are dicult to obtain by this method because the percent retention Varies somewhat with the velocity of the grading liquid impinging on the particles, the type of spray or orifice-employed, the duration and type of brushing used and the method of wetting out or dispersing the material tested. As in the dry method, the most serious causes of error are the variation in diligence between analysts and the judgment of the end point of the operation. The wet methods of the prior art have the additional disadvantage that the grading liquids used, while chosen for their inertness, frequently do have some slight solvent action for the material to be graded. This fact, plus the possibility of attrition of the particles by long brushing or agitation on the sieve, makes it desirable to keep the time of operation of the process to a minimum to prevent inaccurate results.

According to the present invention, I have found that the wet grading of particles may be materially improved and the kdisadvantages of the prior art methods eliminated by placing the particles to be graded on a horizontal screen, rotating the screen at a predetermined rate, and directing a sheet of grading liquid at a predetermined velocity rate substantially along a radius of the screen.

Referring to the accompanying drawing which illustrates a suitable apparatus for carrying out the present invention, Fig. l is a side elevation, partly in section, of a preferred embodiment, and Fig. 2 is a top plan view thereof.

Reference numeral 1 represents a suitable sieve provided with a horizontal circular screen 5 of suitable mesh. The sieve is rotated by variable speed drive 2 and motor 3 controlled by rheostat 4. The grading liquid enters through conduit 7 and is discharged from orifice 8 which may be of any suitable design so as to produce a sheet of liquid 9 which impinges on the particulate material on the screen along a radius of the screen at an angle of from 30 to 90 from the horizontal. The grading liquid passes through the screen into receptacle chamber 10 and may be drawn off through conduit 12 and recycled or otherwise disposed of. Screen 5 may be of any suitable mesh depending upon the size of the particulate material being graded. Three mesh screens up to 400 mesh screens have been satisfactorily used thus grading particles which vary from 6350 microns to as low as 37 microns in size.

In carrying out the present invention a weighed sample of a particulate material to be graded is pasted with a suitable wetting agent to insure dispersion and a measured amount of liquid,` usually water, is added to the slurry and stirred with a high speed stirrer at a denite rate for a delinite period of time. The slurry is transferred to the sieve 1. The motor 3 of the apparatus is started and the speed of revolution of the sieve is regulated to a definite number of revolutions per minute by means of the rheostat 4. A speed of 10 to 20 revolutions per minute has been found to be most suitable. The grading liquid may be any suitable liquid which is a non-solvent for and non-reactive with the material to be tested. Water is usually employed since most pigments and materials tested are pratically insoluble in water. The grading lliquid is supplied through conduit 7 at a definite pre- :rennes1 determined rate of'tlow, usually about 4 liters per minute,

but' the rate may vary widely depending-onthesize-of the apparatus and the size of-f the orifice used. The sheet of water is directedfrom oriiice 8 so as to fall along a radius of thel screen, the" angleb'etween' thfe@ screenja'nd sheet of lWater beir`1`g`"from'30` to' 90.

v It has been foundthat'the velocityof'thesheet'offgrading liquid impinging'on'the particles is important for Vsatisfactory operation of' the device; The velocity should be sufficient to'v properly .agtatev the particles on tlie screen and' break up possible' aggregates:orparticles, but should be low' enough to prevent; spatteringV of Vthe, particles out the analyst. The operation of the apparatus is largely automatic" requiring noN fatiguingmanual shaking by "the operator. The working timerequired is greatly decreased over the time required to test a sample in the prior art.

Most tests are completed in 20 minutes as against an hour or more previously required. The chief advantage however of this invention is that .results may lbe obtained which are notatfected by. thebias oftheanalystand may be reproducedfrom analyst to analyst and from laboral0 torytolaboratory.l

The following data are-typical.othoseobtainediby the apparatus of this invention.

Velocity Volumel Percent Product R. P; M. oi Stream Water; Time, Retention of Sieve Flow, in. liters per Minutes on Sieve per sec. minute No. 325 1 11. 90 t 12.20 1. Lake' made with dichloroben'- 10 70 322 20 11:72l

zidinediazo and-acetoaeeo 11; 70 toluidine. 12. 02, 17.20 15.10 2.ch1oro anthrsquinone `i0 82 3.75 20v 16244- P e r c e n t Reten- V tion .on S i e`v e No. 200.2

. der 3.-MetaNitro-p-toluidine; 10 90 V4.0 20 3. 0163 4. Meta-Nitroptoluidine 20 90 4.0 20 g.

1 U. S. Standard SieveN'o. 325-National Bureau of Standards Letter Circular LC 584. 2'U. S. ctandard SievelvNo. 200 (Ibid).l

of ythe sieve or any possibility of causing significant att`rition of` they particles byabradingthem against the screen. It has been found :that a-velocity of from aboutinches/ second to- 200 inches/ second produces satisfactory results, but-thatoptimum results are secured when'- the velocity'is about 90 inches/ second. The apparatus is allowedto run for? a denite period of timeandlth'en stopped. The operation may be stopped automatically, if desired, by means of ay suitable automatic timer. Ithas been found-'that sieving the particulate material fori minutes yis' about the'optimum time for most-materials'. Thistime oiEY opera"- tion may. be varied. widely solongl as the time is constant for a seriesof testsin whichreplicate values of percentageretention are desired. .It is desirable toakeep` the-test time toy aminimumrocourse, .because of the' desirability of. obtaining rapid results .and the possibility of attrition and solvent .action on thev sample'in` a .prolonged test. The residue .on the screen is driedandweighed .in the-.conventional mannerand its' percent retentionzcalculated.

Theprocesszofr this invention andz'the apparatus .described ,perrnit an analyst, to, obtainreproducible. results on a series of `samples ofa pigmentor on the-same sample in a series of tests. It is possible for theamanufacturer. and user of thepigment to agree on a standard test tobe used in evaluatingl the pigmentsrsold and used. It is merely necessary to stipulate the speedof rotation of the sieve, the velocity of thesheet of ,grading liquid, the time-.of operation,` and such minor details` as the' grading liquid usedar1d the manner of pastingthe material t'obel tested, in ,order to establish astandarrdV test .for any material.

The process and apparatus of` this inventioneliminate all of the troublesome human errors of the priorart grading methods. Each particle on the rotating screen is contacted the samev number ofy times bythe sheet of lgrading liquid thus insuring. uniformity .of action. Mechanical losses .of materialarereduced,to nil because thereI is no possibility for loss throughy-.dust.orspillage The Vdegree andl kind. of .agitation is standardized from v test to. test and 'doesnotfd'ependnon the diligence or peculiarities .of

They tablebelow is givento show how resultsA differed between analysts previously and howV substantiall agreementof results ispossible now by employing this-invention.

Cfomgarsonbf percent retentoiiby the manual wetgrad ing methodond the. method ofy this invention [The; particles graded;y werevthe lake made with Vdichloro-benzidinediazo andaceto-acet-o-toluidine. Sieve No. 325 was used in both methods, andl thefretention'is-in percent of the original sample]` Percent Retention by Method of This Inventionl Percent Retention by Manual Method Analyst Laboratory O @www i 1`A velocity oistreamilow of `90 inches/second was used.

spagaat 5 on the sieve for 30 minutes. It is to be noted that all of the data obtained by analyst #7 in the table above was only one-half as large as that obtained by analyst #6. Also, the data obtained by analysts #1, 2, 7 and 8 are lower than those obtained by the method of this invention while the data obtained by analysts 3, 4 and 6 are higher.

If desired, multiple sheets of liquid directed to impinge along several radii of the rotating screen might be employed in this invention. It is sometimes desirable to employ a nest of sieves with sources of grading fluid above each sieve. In such a case several size gradations are evaluated simultaneously.

I claim:

l. A method of screen testing for particle size analysis whereby the percentage of particles in a given sample of larger than a specified size is determined comprising: weighing a sample of the material to be tested, pasting said sample with a wetting agent in an aqueous phase to form thereby a uniform dispersion of the sample as a slurry, charging said weighed sample to a rotatable cylindrical drum having a horizontal screen at the bottom thereof and contiguous upstanding sides forming a sieve, rotating said sieve at a predetermined rate, directing a sheet of liquid at a predetermined rate of low and pressure and for a predetermined time against the screen at an angle of from 30 to 90 from the horizontal to wash the smaller particles through the screen, whereby the sample particles are classified according to size, the velocity of the sheet of liquid and the rate of rotation of the cylinder being insuicient to cause any appreciable attrition of the sample particles on the screen, drying the residue on the screen, weighing said residue, and calculating the percentage of said residue based on the original sample weight.

2. A process according to claim l in which the velocity of the liquid impinging on the Screen is between about l() inches per second and about 200 inches per second.

3. An apparatus for screen testing for particle Size analysis, whereby the percentage of particles in a given sample of larger than a specified size is determined comprising: a circular screen, contiguous upstanding sides attached to said screen, forming a circular sieve, means to rotate said sieve at a uniform controllable rate, and means to supply a grading liquid at a predetermined rate of ow and at a velocity between 10 and 200 inches per second as a sheet along a radius of the sieve, for a predetermined time, thereby retaining within the sieve all particles too coarse to pass the sieve.

Worrall Oct. 22, 1912 Bullwinkel lune 27, 1939 

1. A METHOD OF SCREEN TESTING FOR PARTICLE SIZE ANALYSIS WHEREBY THE PERCENTAGE OF PARTICLES IN A GIVEN SAMPLE OF LARGER THAN A SPECIFIED SIZE IS DETERMINED COMPRISING: WEIGHING A SAMPLE OF THE MATERIAL TO BE TESTED, PASTING SAID SAMPLE WITH A WETTING AGENT IN AN AQUEOUS PHASE TO FORM THEREBY A UNIFORM DISPERSION OF THE SAMPLE AS A SLURRY, CHANGING SAID WEIGHED SAMPLE TO A ROTATABLE CYLINDRICAL DRUM HAVING A HORIZONTAL SCREEN AT THE BOTTOM THEREOF AND CONTIGUOUS UPSTANDING SIDES FORMING A SIEVE, ROTATING SAID SIEVE AT A PREDETERMINED RATE, DIRECTING A SHEET OF LIQUID AT A PREDETERMINED RATE OF FLOW AND PRESSURE AND FOR A PREDETERMINED TIME AGAINST THE SCREEN AT AN ANGLE OF FROM 30* TO 90* FROM THE HORIZONTAL TO WASH THE SMALLER PARTICLES THROUGH THE SCREEN, WHEREBY THE SAMPLE PARTICLES ARE CLASSIFIED ACCORDING TO SIZE, THE VELOCITY OF THE SHEET OF LIQUID AND THE RATE OF ROTATION OF THE CYLINDER BEING INSUFFICIENT TO CAUSE ANY APPRECIABLE ATTRITION OF THE SAMPLE PARTICLES N THE SCREEN, DRYING THE RESIDUE ON THE SCREEN, WEIGHING SAID RESIDUE, AND CALCULATING THE PERCENTAGE OF SAID RESIDUE BASED ON THE ORIGINAL SAMPLE WEIGHT. 